Exclusion of dysfunctional mitochondria from Balbiani body during early oogenesis of Thermobia

Abstract

Oocytes of many invertebrate and vertebrate species contain a characteristic organelle complex known as the Balbiani body (Bb). Until now, three principal functions have been ascribed to this complex: delivery of germ cell determinants and localized RNAs to the vegetal cortex/posterior pole of the oocyte, transport of the mitochondria towards the germ plasm, and participation in the formation of lipid droplets. Here, we present the results of a computer-aided 3D reconstruction of the Bb in the growing oocytes of an insect, Thermobia domestica. Our analyses have shown that, in Thermobia, the central part of each fully developed Bb comprises a single intricate mitochondrial network. This "core" network is surrounded by several isolated bean-shaped mitochondrial units that display lowered membrane potential and clear signs of degeneration. In light of the above results and recent theoretical models of mitochondrial quality control, the role of the Bb is discussed. We suggest that, in addition to the aforementioned functions, the Bb is implicated in the selective elimination of dysfunctional mitochondria during oogenesis.

Keywords: Balbiani body; Mitochondrial dynamics; Mitochondrial network; Oogenesis.

Fig. 1

Ovariole and morphology of germline cells. a, b Longitudinal sections through the anterior part of an ovariole. c–f Higher magnifications of bouquet stage and previtellogenic oocytes. a, c–f Semithin sections stained with methylene blue; b histocryl section stained with Hoechst 33342. g Fragment of the germarium incubated with MitoTracker and counterstained with Hoechst 33342. Note highly positive Bbs (mitochondrial networks). Individual mitochondria located outside of Bbs display weaker fluorescence. Cystoblast (cb), germarium (g), meiotic oocytes (mo); oocyte nuclei (asterisks), previtellogenic oocytes (po), vitellarium (v), terminal filament (tf), apoptotic germ cell (arrowhead), oocytes in bouquet stage are encircled. Note that Bbs are compact in meiotic oocytes (arrows) and dispersed in previtellogenic oocytes (double arrows)

Fig. 2

Ultrastructure and morphogenesis of the Balbiani body. a–c Cystoblasts (TEM). Accumulations of nuage (nu) are present in the neighborhood of nuclear envelope. Note variously shaped mitochondria located next to nuage accumulations. d, e Bb (Bb) in bouquet stage oocytes (TEM). f–g Degenerating mitochondria (TEM). h Perinuclear cytoplasm of a previtellogenic oocyte (TEM). Cystoblast nucleus (cbn), elements of rough endoplasmic reticulum (rer), Golgi complex (Gc), mitochondria (m), nuage accumulations (nu), oocyte nucleus (on). Note that some mitochondria are associated with nuage accumulations (e, arrows); degenerating mitochondria (d–g, arrowheads)

Fig. 3

Computer-aided 3D reconstructions of Bbs. a, d Two representative 3D reconstructions of the Bb in bouquet stage oocytes. Mitochondrial network (gray), degenerating mitochondria (green), nuage accumulations (blue), Golgi complex (magenta), nuclear envelope (red). b, c, e, f Reconstructions showing central parts of (a) and (d), respectively. For these reconstructions, only 3 serial sections have been used. For clarity of the images, colors of subsequent (deeper and deeper) levels (representing subsequent sections) have been depicted darker

Fig. 4

Computer-aided 3D reconstructions of dispersed Bbs. a, b Two representative 3D reconstructions of the mitochondrial networks in previtellogenic oocytes. Mitochondrial network (gray), elements of RER (yellow), nuclear envelope (red). For clarity of the images, colors of subsequent (deeper and deeper) levels (representing subsequent sections) have been depicted darker

Fig. 5

Apoptosis of germline cells. a Fragment of the germarium. TUNEL assay. Note apoptotic cells (arrows). b, c Fragments of apoptotic oocytes (TEM). Note altered morphology of Bb: dense mitochondria (m), transparent and swollen nuage accumulation (snu) and distended cisternae of Golgi complex (dGc). Elements of RER (rer), oocyte nucleus (on); blebs of the nuclear envelope filled with heterochromatin (b, asterisk)